1. Field of the Invention
The present invention relates to organic light-emitting devices. More particularly, this invention provides highly luminance-efficient and bright organic light-emitting devices, and organic light-emitting devices with adjustable emission zone.
2. Description of the Related Arts
Recently, with the development of multimedia technology and the coming of information society, the requirement to high performance flat panel display becomes more and more emphasized. The recently developed three kinds of display technology, i.e. plasma display, field emission display and organic light-emitting display, can make up shortcomings of the CRT and LCD to a certain extent. Among them, organic light-emitting devices (OLEDs) show many advantages such as self-emission, low voltage operation, all-solid state, wide view angle, and full-color. The OLEDs also show a quick responding speed of up to 1000 times that of the LCD display and its manufacturing cost is relatively low comparing with the LCD display with same distinguishability. Thus, OLEDs show a great foreground in the display field.
In 1987, C. W. Tang et al. of Kodak Company reported a light-emitting diode with a double-layer structure of organic thin films, which is prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium-tin-oxide anode and an alloyed Mg:Ag cathode. High external quantum efficiency (1% photon/electron), luminous efficiency (1.51 m/W) and brightness ( greater than 1000 cd/m2) were achievable at a drive voltage of 10V. (C. W. Tang, Applied Physics Letters. 51, 913 (1987)) In 1990, Burroughes et al. of Cambridge University found that polymer material showed excellent electroluminescent characteristics and fabricated the first polymer light-emitting diodes, thus extending the development of organic light-emitting diodes to polymer field. In the past ten years, many efforts have been made to improve the device performance.
It was recognized that OLEDs efficiency strongly depends on balance of holes and electrons. In the conventional NPB/Alq3 OLEDs, mobility of the holes in NPB is much larger than mobility of the electrons in Alq3, leading to unbalance of the holes and electrons in the emission zone.
It has been reported that using other appropriate hole-transporting materials or adopting novel device structure configurations is a good solution to enhance performances of the OLEDs. As one of the effective methods for reducing the hole mobility, it is tried to dope rubrene in hole-transporting layer in order to increase the OLEDs performances by Y. Hamada et al. and M. S. Jang et al. (Y. Hamada, T. Sano, K. Shibata, and K. Kuroki, Jpn. J. Appl. Phys., Part 2 34, L824 (1995); M. S. Jang, S. Y. Song, H. K. Shim, T. Zyung, S. D. Jung, L. M. Do, Synth. Met. 91, 317 (1997)) Aziz et al. proposed that the enhancement in these OLEDs performances is caused by the presence of rubrene molecules which act as hole traps in the devices. (H. Aziz, Z. Popovic, N. X. Hu, A. M. Hor, and G. Xu, Science 283, 1900 (1999); H. Aziz and Z. D. Popovic, Appl. Phys. Lett. 80, 2180 (2002)) On the other hand, organic multiple-quantum-well (MQW) structures have demonstrated to be helpful for obtaining narrower spectral emission, higher emission efficiency and tunable emission spectrum. But in the recent research works, all the MQW structures were introduced to enhance the concentration of the holes and electrons in the emission zone, resulting in high performances. N. Tada et al. (N. Tada, S. Tatsuhara, A. Fujii, Y. Ohmori and K. Yoshino, Jpn. J. Appl. Phys. 36, 421 (1997)) used Alq3/TPD MQW structures in the emission layer and obtained a higher luminance efficiency compared with the conventional double-layer devices. Some further works showed that the enhancement was attributed to the increase of the concentration of carriers and the confinement of excitons in the emission layer. However, by using MQW structure in the light-emitting layer to increase the concentration of carriers, there is still an imbalance of carriers in the light-emission zone. And the superfluous holes will decrease the emitting efficiency. Thus it is relatively limited for enhancing the device efficiency by introducing MQW structure in the emission layer.
An object of the present invention is to provide highly luminance-efficient and bright organic light-emitting devices.
Another object of the present invention is to provide organic light-emitting devices with adjustable emission zone.
These objects are achieved by an organic light-emitting device comprising a transparent substrate, a first electrode layer, a second electrode layer, a hole-transporting layer and a electron-transporting layer sandwiched between the first and second electrode layers, wherein the hole-transporting layer includes organic multiple-quantum-well structure, and the multiple-quantum-well structure has a period number of alternating layers formed of a layer of organic material A with wide energy gap and a layer of organic material B with narrow energy gap. Organic material A and organic material B are selected such that their energy levels agree with the following relationships (I) and (II):
(I) highest occupied molecular orbital (HOMO) levels of organic material A are lower than those of organic material B;
(II) lowest unoccupied molecular orbital (LUMO) levels of organic material A are higher than those of organic material B.
The period number in the present invention is desirably an integer of 1 to 10.
The energy levels of the two organic materials in the hole-transporting layer in the present invention match each other (in agreement with the above relations (I) and (II)), thus the energy level of the organic material A can enwrap the energy level of the organic material B. Because the carriers at the interface of the two materials trend to move towards the low energy directions, thus the holes and electrons trend moving to the organic material B layer. Thus there is an energy gap for carriers at the interface of the organic material A and material B and the well of the electrons and holes are all in the organic material B. When the holes transport through the MQW structure, the majority of the holes distribute in the organic material B, and a few holes distribute in the organic material A. Furthermore, the holes can only transport through the organic material A by tunneling injection and consume some energy due to the existence of the hole energy barriers at the interface. Thus, it can be concluded that: (1) the energy consumed when the holes go through the interface increases with increase of the energy barrier at the interface. Thus more holes without enough energy will be confined in the organic material B and can""t pass through the MQW structure; (2) with increase of the period number, the carriers have to traverse through more interfaces for going through the MQW structure. And more holes will be blocked at the interface, thus less holes can arrive to the emission zone. Therefore, the hole transport in the hole-transporting layer can be well controlled, and the balance injection of the hole and electron carriers can be obtained by choosing appropriate materials and period number of the MQW structure, resulting in high efficiency and luminance of the light-emitting devices.
The organic material B in the present invention can also be a kind of dye material C.
It was recognized that in the MQW transporting structure, (1) the carriers need to consume more energy to pass through the interfaces with the increase of the energy barriers therein, more carriers cannot pass through the MQW structure and will be confined in the organic material C due to their low energy and (2) the carriers need to pass through more interfaces with the increase of the period number, thus more carriers are blocked at the interfaces and cannot pass through the MQW structure either. Therefore, when the energy barrier at the interface of the MQW structure is small enough ( less than 0.4 eV), the majority of the holes will be confined in the MQW structure, but a fraction of the holes can still transport through the MQW structure. At the same time, a fraction of the electrons recombine with the slight (i.e. small quantity of) holes in the electron-transporting layer, and the residual electrons transport into the MQW structure and recombine with the majority of the holes. Thus two emission zones will be obtained. By changing the period number of the organic MQW structure, the emission zone in the devices can be adjusted in the electron-transporting layer (period number is small enough and most of holes transport to electron-transporting layer and recombine with the electrons), hole-transporting layer (period number is large enough and the holes are all confined in MQW structure), or both the hole-transporting and electron-transporting layers. In one word, the recombination and emission zone are adjustable by controlling the period number of the organic MQW structure.
A buffer layer can be sandwiched between the first electrode and hole-transporting layer in the present invention, and a transitional layer can be sandwiched between the hole-transporting layer and electron-transporting layer.
The organic light-emitting devices in the present invention have below advantages:
1. The mobility of holes carriers in hole-transporting layer can be efficiently adjusted by the employment of the MQW structure. And hole-electron injection balance and high luminance efficiency can be obtained.
2. The devices show different emission zone as the period number changes when the organic material B with narrow energy gap is replaced by dye material C. Thus, the hole-electron recombination zone and emission zone of the organic light-emitting devices can be adjusted by controlling the period number of the organic MQW structure in the hole-transporting layer.